This invention pertains to systems for passively determining the direction of arrival and range of radiant energy emitted by a distant, scanning source, such as a radar transmitter.
While systems in accordance with the invention can be, in general, used with many different types of scanning, radiant energy sources, such as electromagnetic radiation including radio frequencies, visible and invisible light, etc., and such as mechanical wave energy including sound, it is particularly useful for locating scanning sources that emit radio frequencies such as radar transmitters. Passive-type range and direction determining systems involve locating the source of the radiant energy by receiving the transmission of such energy, developing electrical signals representative thereof and processing such signals to yield the desired information, without requiring a change in position of the receiving station. While the range and direction information can be readily obtained by using standard triangulation techniques involving the movement of the receiving station between two or more different locations, such maneuvers of the receiving station are at best time consuming, and in a military environment, strategically undesirable.
For these reasons, passive-type systems have been developed in which the source radiation can be located both in terms of direction and range at any given instance. The measurements may be made from a receiving aircraft, while in flight, in which case the direction and range information is continuously changing as the receiving aircraft and source are moving relative to each other. In one known passive-type system, pertinent to the present invention, the range measurement is made by deploying a pair of spaced-apart receiving antennas, such as may be mounted on the wing tips of the receiving aircraft, and then measuring a time differential t.sub.21 resulting from the slight time difference in the receipt of the source radiation as it scans broadside of the pair of spaced-apart receiving antennas. The measured time differential t.sub.21 together with the detected scan rate .omega..sub.s, and the known geometry of the receiving antennas relative to the distant scanning source, provide sufficient information for automatically and electronically measuring the range R.
The simplest geometrical configuration for such a system involves a source that is located along the perpendicular bisector of an imaginary line that extends directly between the spaced-apart receiving antennas. In such case, the relationship between the measured time differential t.sub.21 associated with the receipt of the scanning source at the spaced antennas, the source scan rate .omega..sub.s and the range R is easily derived. However, in most actual systems, the source may arrive at the receiving antennas at an angle of arrival .phi. that has some nonzero value with respect to the above-mentioned perpendicular bisector. It is thus necessary for the range measuring system to incorporate a separate, dedicated subsystem for measuring the angle of arrival of the source RF radiation, and to compensate the measured time differential in accordance with the magnitude of the measured angle of arrival to obtain the correct range measurement.
One common way of implementing this angle of arrival measurement is to employ a circularly scanning receiving antenna coupled to conventional processing circuitry for determining the angular position of such antenna when the received signal is at maximum strength. The operation of such a subsystem produces an electrical signal representing the angle of arrival .phi.. Further signal processing is used to develop the proper trigonometric function of the angle .phi. such that when multiplied or otherwise combined with the measured time differential t.sub.21 (during the signal processing that determines the range R) the system automatically compensates for all relative orientations of the source.
An example of a range determining system of the foregoing type is disclosed in application Ser. No. 488,401 entitled SERVO-LOOP PROCESSOR, filed July 15, 1974, by Philip Jones and Moorfield Storey, Jr., now abandoned, and a continuation-in-part of such application, copending as application Ser. No. 80,533 entitled SERVO-LOOP PROCESSOR, filed Oct. 1, 1979, now U.S. Pat. No. 4,316,193.
With this background, it is an object of the present invention to provide a method and apparatus for measuring the range to a scanning source of radiant energy in which the means and steps of performing the angle of arrival measurement are integrated with the same signal receiving and processing circuitry and method steps that yield the range measurement.
Another object of the invention is to eliminate the need for a separate, dedicated angle of arrival measuring subsystem in a range determining system for locating scanning emitters.
An additional object is to provide an accurate, versatile and reliable method and apparatus for determining the angle of arrival of received radiant energy emitted by a scanning source.